1. Field of the Invention
This invention relates to an electrophotographic image forming apparatus such as a copying machine and a printer, and more particularly, to an image forming apparatus that accomplishes electrical charging by a contact type charging device.
2. Description of the Related Art
Conventionally, in an image forming apparatus utilizing an electrophotographic method such as a copying machine and a printer, the photoreceptor is charged by a charging device and an electrostatic latent image is formed by image exposure in that charged region. This latent image is developed, made a visible image and then transferred to the transfer member and fixed.
Although various types of charging devices have been known, if broadly classified, they can be divided into corona charging devices which utilize corona discharge and contact type charging devices which force fixed type charging brushes, rotational roller type brushes, endless belt type brushes, and rotational rollers, etc. to make contact with the surface of the photoreceptor.
Though the charging devices which utilize corona discharge have an advantage such that stable charging can be carried out, problems exist in which large amounts of ozone generate while corona discharging is carried out with this causing the photoreceptor to deteriorate as well as having harmful effects on human bodies. Thus, contact type charging devices which generate remarkably low amounts of ozone compared to the amounts of ozone generated by corona charging devices are attracting attention.
Even among these contact type charging devices, in particular, charging devices utilizing a rotational brush are attracting attention from the viewpoint of stable charging.
Furthermore, using a voltage that includes an AC component for charging voltage applied to the charging device is attracting attention from the viewpoint of stabilizing the charging without being affected by variations in the environment. For example, Japanese Laid-open Patent Application No. 63-9233 discloses the application of a voltage to a charging roller in which an AC voltage is superimposed on a DC voltage.
According to the above conventional art, from both viewpoints of suppressing ozone gas and stabilizing the charge, using a rotational brush as the charging member and a voltage which includes an AC component as the charging voltage is preferable.
However, according to the research of the inventors of this invention, in a laser printer with a resolution of, for instance, 400 dpi, 600 dpi or an even higher resolution, if a shading pattern image (also called half image) is formed utilizing a rotational brush as the charging member and utilizing a voltage that superimposes an AC voltage on a DC voltage as the charging voltage, moire-shaped image noise will appear in the formed image.
The moire-shaped image noise stated here is such an image noise that a band shaped portion which seems to have dark image density repeatedly appears. FIG. 20 shows one example of this. The image noise shown in FIG. 20 is image noise in which dots in a certain range fatly developed when a shading pattern image like the one shown in FIG. 19 is formed utilizing a rotational roller-type brush as the charging member and utilizing a voltage that superimposes an AC voltage on a DC voltage as the charging voltage in a laser printer, and the collection of these fat developed dots seem darkly in a band shape as a whole and this band-shaped portion N repeatedly appears.
This type of moire-shaped image noise is not comparatively noticeable at a low resolution of, for example, 240 dpi but becomes more noticeable as the resolution increases. This problem must be solved with today's increasingly higher resolution images.